1. Field of the Invention
This invention relates to a television camera using a solid state image sensing device such as a charge coupled device for producing a color television signal and in which digital processing is performed.
2. Description of the Prior Art
A color television camera has been proposed in which an image pickup output (an analog signal) derived from the solid state image sensing device is subjected to a digital intermediate process and a standard analog color television signal is finally produced. As compared with another conventional color television camera in which a color television signal is analog-processed over the whole signal process interval, the aforesaid color television camera is particularly superior in signal processing, circuit construction, and reliability. Accordingly, so as to interface with other digital equipments, it is desirable to use digital signal processing in color television cameras.
In using digital-processing of the image pickup output for producing a color television signal, the processing rate for producing a digital modulated color signal is normally selected to have a frequency which is three or four times the color subcarrier frequency f.sub.sc. This is because the color subcarrier frequency is f.sub.sc in the standard color television signal, as well as the band width of the video signal, and for easy processing of the signal when the video signal is considered as two-dimensional sampling system.
Accordingly, when 3f.sub.sc or 4f.sub.sc is selected as the frequency of a reference clock, circuit systems for digital signal processing must be all driven at a rate of 3f.sub.sc or 4f.sub.sc, for example, the frequency of the driving pulses for driving a solid state image sensing device, the sampling rate of a sampling pulse for sampling-holding the output of the solid state image sensing device, the processing rate for converting the image sensing output from its analog form to digital form, and the like. However, if such reference clock frequency is used for processing in the circuit system, the frequency will exceed 10 MHz which increases the processing speed higher than it need be in the circuit system. Further, if a cut-off frequency is constant as in digital filters used for limiting the frequency band of a color signal, for example, the number of delay elements or the like used in such digital filters increase proportional to the processing rate and a digital processing circuit having many components would be required resulting in high cost and complicated integrated circuits.